A ‘panic’ response is a normal physiological survival reflex in humans and can be elicited by either an exteroceptive or interoceptive cue perceived as life-threatening. Panic disorder is characterized by recurrent episodes of severe anxiety accompanied by multiple physical symptoms such as increased cardiorespiratory responses. Panic disorder is also a risk factor for suicidal behavior.
The initial pathology in patients with panic disorder appears to be an alteration in central neural pathways regulating normal panic responses, thus rendering the patients susceptible to unprovoked panic symptoms when exposed to ordinarily mild stressors.
Panic attacks can be reliably induced in panic disorder patients in the laboratory by specific, normally innocuous interoceptive stimuli (e.g., intravenous 0.5M sodium lactate or yohimbine, or 7% CO2 inhalations). These induced attacks are similar to spontaneously occurring episodes that characterized by sudden onset of fear symptoms along with rapid increases in respiration and heart rates. This indicates that global neural pathways that modulate arousal are perturbed in these patients. Consistent with this, reduced central GABAergic activity has been reported in subjects with panic disorder and drugs that restore GABAergic inhibition (e.g. benzodiazepines) have been used as treatments. Furthermore, acute disruption of GABAergic inhibition in panic-generating CNS sites such as the dorsomedial/periformical hypothalamus, amygdala or the dorsal periaqueductal grey leads to panic-like behavior and increased cardiorespiratory responses in rats. After chronically inhibiting GABA synthesis in the dorsomediaUperiformical hypothalamus of rats with 5 days of local 1-allylglycine (1-AG: a GABA synthesis inhibitor) infusions (using osmotic minipumps connected to a cannula directed at dorsomedial/periformical hypothalamus), sodium lactate challenges produce anxiety (measured by social interaction, elevated plus maze, open field test, and freezing in defensive probe burying test) as well as panic (characterized as increased “flight”-like locomotion and increased heart rate, mean arterial pressure responses). This is also pharmacologically validated with anti-panic drugs such as alprazolam, and provides a robust animal model of human sodium lactate-induced panic attacks.
Orexins (ORX), also called hypocretins (Hcrt), are neuroactive peptides that are produced by neurons located in the dorso-medial periformical and lateral hypothalamic areas of the brain. Orexin A and orexin B are hypothalamic peptides derived from a common precursor polypeptide called prepro-orexin. Human prepro-orexin mRNA encodes a 131-residue precursor peptide (prepro-orexin). The human prepro-orexin gene consists of two exons and one intron distributed over 1432 base pairs. The 143-base pair first exon includes the 5′-untranslated region and a small part of the coding region that encodes the first seven residues of the secretory signal sequence. The second exon contains the remaining portion of the open reading frame and 3′-untranslated region. Human pre-pro orexin mRNA has been characterized by Sakurai et al, J. Biol. Chem. 274(25): 17771-17776 (1999), the content of which is herein incorporated by reference in its entirety.
Prepro-orexin is processed to form pro-orexin, which is further processed to form orexin A and orexin B. Orexin A is a 33-amino acid peptide of 3562 Da with two sets of intrachain disulfide bonds. It has an N-terminal pyroglutamyl residue and C-terminal amidation. The primary structure of orexin A predicted from the cDNA sequences is completely conserved among several mammalian species (human, rat, mouse, cow, sheep, dog, and pig). On the other hand, rat orexin B is a 28-amino acid, C-terminally amidated linear peptide of 2937 Da that is 46% (13/28) identical in sequence to orexin A. The C-terminal half of orexin B is very similar to that of orexin A (73%; 11/15), whereas the N-terminal half is variable.
Orexin A and Orexin B are endogeneous peptides that activate orexin receptors, for example, orexin receptor type 1 (OX1R) and orexin receptor type 2 (OX2R), which are G-protein coupled receptors. Stimulation of these receptors by orexins causes an increase in intracellular calcium levels in hypothalamic cells in vitro. These hypothalamic neurons are the origin of an extensive and divergent projection system innervating numerous structures of the central nervous system.
ORX producing neurons in the dorsomedial/periformical and lateral hypothalamus and are known to regulate feeding, wakefulness and vigilance. It has been discovered herein that ORX neurons are involved in mobilizing sympathetic responses and desensitizing the parasympathetically mediated baroreflex to permit simultaneous increases of blood pressure and heart rates, which are all components of panic. These autonomic nervous system targets of ORX neurons are activated by sodium lactate infusions in sodium lactate panic prone rats but not in controls. Mice lacking the prepro-ORX gene have attenuated defense responses to panic cues and cardioexcitatory responses following disinhibition of the dorsomedial/periformical hypothalamus.
Acute hypercapnia (elevated arterial CO2), rapidly increases extracellular pH when elevated levels of plasma CO2 combine with water to form carbonic acid. Hence, the concentration of CO2 in the blood is highly regulated and maintained within a very narrow range. Mild elevations of CO2 initially increase respiration rate and tidal volume to help “blow off” excess CO2. However, as CO2 levels continue to increase, additional physiologic responses are initiated, including adaptive autonomic, behavioral and neuroendocrine responses. For instance, exposing rats to mildly elevated concentrations of hypercarbic gas (e.g., 7% CO2) results in increased respiration rate and tidal volume that serve to reduce partial pressure of CO2 (PCO2) without mobilizing other components of the “panic-like” response. However, exposing rats to higher concentrations of hypercarbic gas (e.g., ≧10% CO2) elicits additional components of a full blown panic-like response as evidenced by increases in sympathetic activity, hypertension, anxiety-like behaviors and mobilization of the hypothalamic-pituitary-adrenal (HPA) axis.
Acute hypercapnia (elevated arterial CO2) can be life-threatening and rapidly mobilizes adaptive changes in breathing and behavioral arousal in order to restore acid-base homeostasis. Severe hypercapnia, seen acutely in sleep disorders (e.g., sleep apnea) or chronically in respiratory disorders (e.g., chronic obstructive pulmonary disease, COPD), also results in high anxiety and autonomic activation. Recent evidence has demonstrated that hypothalamic orexin (ORX: also known as hypocretin) neurons, which help to maintain waking states and vigilance, are Sensitive to local changes in CO2/H+ through acid-induced closure of leak-like K+ channels, and mice lacking prepro-orexin have blunted respiratory responses to hypercapnia.
Severe hypercapnia-induced autonomic hyperactivity and anxiety responses are relevant to managing hypercapnic conditions such as chronic obstructive respiratory disease (COPD), obstructive sleep apnea syndrome (OSAS), sudden infant death syndrome (SIDS), congestive heart failure, emphysema, asthma, bronchitis, pneumonia, cystic fibrosis, and alpha-1 antitrypsin, deficiency. In humans, even a single breath of air containing 35% CO2 mobilizes sympathetic-adrenal responses and increases anxiety-like symptoms. However, the mechanism by which high CO2 levels elicit panic-like responses is heretofore unknown.
Although the carotid body is the primary peripheral CO2/pH chemoreceptor, CO2 readily crosses the blood-brain barrier to directly interact with central chemoreceptive neurons. Specialized CO2/H+ chemosensory neurons with a high chemosensitivity (˜300%, ˜110% or ˜120% increase in firing rate with 0.1 unit extracellular pH change, respectively) are found in medullary regions such as the retrotrapezoid nucleus, medullary raphe, and ventrolateral medulla. Without being bound by theory, it is believed herein that medullary chemosensitive neurons are important for regulating breathing following subtle changes in CO2/H+ due to their proximity to major cerebral arteries and the brain surface. The ORX producing neurons, which are localized to the dorsomedial/perifornical (DMH/PeF) and adjacent lateral hypothalamus (LH) also display CO2/H+-sensitive properties, but with lesser chemosensitivity (˜100% increase in firing rate with 0.1 unit extracellular pH change).
Subjects with chronic episodes of hypercapnia (such as patients suffering from a chronic pulmonary disease including asthma, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, cystic fibrosis, and sarcoidosis) have significant co-morbidity with severe anxiety and sympathetic arousal, both of which can make management of these patients difficult. It is discovered herein that the orexin system plays an important role in responses to hypercapnia, particularly with concomitant severe anxiety. Current treatments of anxiety, such as fast acting benzodiazepine drugs, are not ideal for treating anxiety associated with hypercapnic conditions due to significant respiratory depression and other peripheral side effects. Thus, new therapies that can reduce or alleviate symptoms associated with panic disorder, anxiety, and hypercapnic conditions are desired.